Resin-soluble thermoplastic veil for composite materials

ABSTRACT

A resin-soluble thermoplastic polymer veil toughening element for a curable composition wherein the polymer element is a non-woven veil in solid phase adapted to undergo at least partial phase transition to fluid phase on contact with a component of the curable resin matrix composition in which it is soluble at a temperature which is less than the temperature for substantial onset of gelling and/or curing of the curable composition and which temperature is less than the polymer elements melt temperature; a method for the preparation thereof, a preform support structure for a curable composition comprising the at least one thermoplastic veil element together with structural reinforcement fibers, methods for preparation thereof, a curable composition comprising the at least one thermoplastic veil element or the support structure and a curable resin matrix composition, a method for preparation and curing thereof, and a cured composite or resin body obtained thereby, and known and novel uses thereof.

This application, Ser. No. 11/429,765, claims priority to theprovisional application 60/679,114 filed May 9, 2005. Related cases,PCT/US06/17604 and PCT/US06/17915 also claim priority to the provisionalapplication 60/679,114.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to the manufacture and use of non-woventhermoplastic veils for use in complex and diverse high performancecomposite applications. In preferred embodiments, this invention relatesto a resin-soluble thermoplastic polymer formed into a non-woven veilfabric or random mat for incorporation into high performance compositemanufacturing applications to aid part manufacturing and improve desiredproperties.

2. Description of the Related Art

Fiber-reinforced resin matrix composites are widely accepted for use ashigh strength low weight engineering materials to replace metals inaircraft structural applications and the like. These curablecompositions may be made by laminating prepregs comprising high strengthfibers, such as glass, quartz, graphite (carbon), boron, aramid or thelike impregnated with an epoxy resin matrix. Important properties ofsuch composites are high strength and stiffness with reduced weight.

Curable compositions comprising a blend of polymer resins andreinforcing fibers are characterized by individual physical and chemicalproperties of the constituent polymer resins and fibers, wherebycompositions may be selected for a specific use. Typically, therefore athermoset resin component is present which confers high solventresistance, thermal cycling resistance, etc. In addition, a thermoplastresin component may be present to confer higher levels of toughness,fire resistance, etc, and reinforcing fibers to confer high levels ofstiffness, strength, etc.

Composites are traditionally made using curable compositions known asprepregs made up of fiber reinforcements or structural fabricsimpregnated with a resin matrix. Sheets of prepreg may be cut to sizefor laying up and molding and laminating in the construction of a givencomposite structure. Prepreg properties and resulting part quality canbe controlled to manipulate resulting part properties such as toughness,strength, flexibility and the like.

Prepregs traditionally contain epoxy resins impregnated on fiberreinforcements. Should the resulting part require enhanced propertiessuch as additional toughness, additives such as thermoplastics may beadded to the epoxy resin. However, use of prepregs results in certaindisadvantages including labor costs, difficulty in forming complexshaped parts, controlling toughened locations, and increasedmanufacturing costs due to the use of automated tape laying or fiberplacement equipment and autoclaves for curing.

Recently there has been an emergence of an alternative technology formanufacturing composite parts, which technology is generally referred toas resin infusion. This approach differs from that of conventionalprepreg technology in that dry structural reinforcement fibers arearranged in a mold as a preform, which consists of one or more layers orplies of dry oriented structural reinforcement fiber material assembledin a stack. Then the preform is then injected or infused directlyin-situ with the resin matrix.

Resin infusion is a generic term which covers processing techniques suchas Resin Transfer Molding (RTM), Liquid Resin Infusion (LRI), VacuumAssisted Resin Transfer Molding (VARTM), Resin Infusion with FlexibleTooling (RIFT), Vacuum Assisted Resin Infusion (VARI), Resin FilmInfusion (RFI), Controlled Atmospheric Pressure Resin Infusion (CAPRI),VAP (Vacuum Assisted Process) and Single Line Injection (SLI).

The potential benefits that resin infusion has to offer over that of aconventional prepreg route are reduced scrap, reduced lay-up time,reduced capital cost, a non-dependence on tack and drape and increasedshelf life properties. In practice, the use of resin infusion technologyfinds its greatest use in specialized operations requiring complexcomposite structures, localized toughening, and very large structures,such as in aircraft wings and fuselages, marine or wind applications.

When the preform is placed in a mold, the layers or plies are typicallyheld in place, stabilized and compacted/debulked, by stitching, staplingor bonding using binders and tackyfiers. These operations maintain theorientation of the fibers and to stabilize the preform in order tomaintain the geometry and the dimensions of the preform and to preventfraying or the pulling apart of the dry perform during storage,transport and handling.

The preform may then be carefully cut outside of the stitching orstapling to a desired shape. The preform is then placed in a mold andresin injected to impregnate the fabric. The infused preform is thencured by ordinary and accepted procedures to provide a finishedcomposite structure.

However, stitching and stapling for preform stabilization are typicallylimited as the preform cannot be shaped to conform to a complexstructure's contour without disturbing the stitching or stapling.

One means of overcoming the stitching problem has been through the useof weaving a thermoplastic into the structural reinforcement fibers,which thermoplastic will melt when slightly heated to stabilize thepreform, and then will melt into the epoxy resin matrix during cure. SeeU.S. Pat. No. 4,741,073, for example. However, this practice is limitedin the amount of thermoplastic that can be added, requires highprocessing temperatures to ensure complete melting of the thermoplasticinto the epoxy resin matrix.

While resin infusion technology is promising, composites which have highimpact requirements usually contain thermoplastic toughening agents andit is difficult to add a thermoplastic toughening agent to an injectableresin because thermoplastic toughening agents possess such highmolecular weight that they greatly increase the viscosity of the resin.Therefore, only small amounts of a thermoplastic toughening agent can beadded to the resin of these resin infusion systems.

A potential way to efficiently provide thermoplastic toughening agentsfor a resin infusion system is to not add the thermoplastic to theresin, and to introduce it in some other way to the preform. In the caseof resin infusion technology, one means of overcoming this limitationhas been by introducing the thermoplastic polymer by weaving it directlyinto the carbon fibers.

EP 392939 discloses a method of preparing a traditional prepreg withreinforcing fibers co-woven with thermoplastic fibers which melt. Thesesystems do not however attempt to introduce an additional resin matrixinto the prepreg, as in resin infusion systems, and typically employvery high molecular weight thermoplastic polymers, which requireexcessively high temperature and pressure to melt.

One early attempt at incorporating a thermoplastic fiber into carbonfiber fabric for resin infusion is disclosed in U.S. application Ser.No. 10/381,540. This application discloses the use of a thermoplasticfiber woven within the structural reinforcement fiber for use in a resininfusion preform system. However, this application discloses that thethermoplastic fibers remain in the final composite, after curing ofcomponent, i.e., the thermoplastic fibers do not dissolve in the resin.

A related attempt to overcome these issues in resin infusion technologyis to provide a preform which introduces a flexible thermoplasticpolymer fiber with the reinforcing fibers by weaving the thermoplasticpolymer fiber with the reinforcing fibers. The flexible thermoplasticpolymer fiber is in a solid phase and adapted to undergo at leastpartial phase transition to fluid phase on contact with a resin matrixcomponent of the curable composition at a temperature that is less thancure temperature for the curable composition and which is less than themelting temperature of the flexible thermoplastic polymer fiber. Seepublished US Patent Application No. US 20040041128 assigned to CytecTechnologies, Inc. and published Mar. 4, 2004.

While this method of co-weaving a thermoplastic fiber with thereinforcement fibers overcomes many of the problems of traditional resininfusion technology, it does not provide the capability of localizedtoughening. The thermoplastic fibers are woven throughout thereinforcement fibers and such that there is an even distribution offibers and resulting even concentration of thermoplastic throughout thecomposite part. Therefore, this method has limited flexibility in theamount of added thermoplastic polymer that may be incorporated into thecomposite part. There is no provision in this co-weaving technology toconcentrate additional amounts of thermoplastic polymer for increasedtoughening at a particular location of the composite part.

Additionally, reinforcement fiber fabrics co-woven with thermoplasticfibers concentrate the thermoplastic toughening agent predominantlywithin the same plane as the reinforcement fibers rather than betweenlayers where it may be more preferred. The toughening agent is preferredto be primarily situated between structural reinforcement fiber layers,rather than within, to carry the higher stresses incurred between thereinforcement fiber layers of the final composite.

Moreover, the thermoplastic fibers are not practically able to be addedto unidirectional tape due to the high tension required to manufactureunidirectional carbon fiber tape.

The incorporation of the thermoplastic toughening agent as a fiberco-woven into the structural reinforcement fiber further led to somedifficulties in controlling amount of thermoplastic toughening agent inthe resin. This control is preferred in order to match data for currentresin formulations containing precise amounts of thermoplastictoughening agent. In order to obtain the appropriate amount ofthermoplastic polymer, the number of thermoplastic fibers and uniformityof distribution throughout the structural reinforcement preform had tobe controlled. The number, size and placement of the thermoplasticfibers within the preform had to be controlled to allow for the uniformdistribution of the thermoplastic polymer to result in the duplicationof current toughened resin formulations.

The co-weaving method also results in increased manufacturing costs dueto the requirement of spinning the thermoplastic polymer into a fibercapable of being woven with the structural reinforcement fibers.Moreover, the added complexity of co-weaving a dissimilar material suchas a thermoplastic fiber with the structural reinforcement fibersincreased manufacturing costs.

Additionally, co-weaving of the thermoplastic fiber disturbed thearrangement and straightness of the structural reinforcement fibers,thus reducing in-plane mechanical properties and did not afford theinterlaminar toughening that was desired of composite systems. In otherwords, the thermoplastic polymer is preferred to be concentrated morehighly between the structural reinforcement fiber layers rather thanwithin.

A modified attempt to overcome these shortcomings of co-woven toughenedresin infusion systems has been to interpose thermoplastic interleaflayers between reinforcement structural fiber layers. See U.S. Pat. Nos.6,437,080B1, 5,288,547, EP 1 473,132 A2 and EP 0327,142 A. However, thisinterleaf toughening material does not dissolve into the resin, butrather melts at the higher temperatures attained during cure providinglimited diffusion and requiring higher temperatures and cure times toadvance the melting and diffusion.

Melting interleaf layers are limited in that it is difficult tomanufacture quality components by RIFT or VARTM, out of the autoclave,without the benefit of high temperatures and cure times needed toadvance the melting and diffusion in these systems. Curing with vacuumonly or with no pressure causes the components to have very high voidcontent thus, leading to poor mechanical properties.

Melting interleaf systems are further limited in that the lack ofuniform and complete dissolution creates interfaces between the resinand reinforcement fibers. This interface can in turn decrease compositeresistance to fluids and cause a reduction in hot/wet mechanicalproperties. Furthermore, the lack of complete dissolution of thethermoplastic and the resin means that no synergy between the propertiesof the two chemical species can be obtained; for example the resin willmaintain its brittleness and the thermoplastic will remain sensitive tosolvents.

Other means of incorporating thermoplastic tougheners into resininfusion systems include powder coating the dry fibers or the fabricwith a particulate thermoplastic material such that when the dry fibersor the fabric are laid up, slight heating of the fibers melts theparticulate thermoplastics and fuses and stabilizes the plies.Additionally, the thermoplastic particles melt at the cure temperatureand diffuse into the resin system, toughening the matrix. However,melting and diffusion of these thermoplastics into the resin matrix alsorequires the high temperature curing processes and is limited in theamount and extent of diffusion of the thermoplastic. See U.S. Pat. No.5,057,353 which uses thermoplastic particles on the surface of the resininfused prepregs to add toughening properties. This system is an epoxyresin impregnated prepreg rather than a coated dry fiber applicable forresin infusion. Furthermore, the thermoplastic particles are intended tomelt upon curing and do not dissolve prior to attaining higher curetemperatures necessary to melt them. Another disadvantage of particletoughening is that during resin infusion, the particles can be washedaway by the resin flow and can uncontrollably agglomerate at undesirablelocations. This causes the mechanical properties of the composites to benot uniform and may cause undesirable voidage and porosity due tovarying viscosities of the resin causing a non-homogeneous flow front.

Other methods of incorporating thermoplastic particles or fibers intoreinforcement structural fibers are disclosed in EP 0842038 B1; WO03/038175 A1; JP 2000119952 A2 and U.S. Pat. No. 6,060,147.

However, localized toughening of composites remains difficult usingthese techniques during manufacture because the toughening agent isuniformly coated onto the reinforcement fibers and cannot becontrollably increased in any particular area of the part that mayrequire increased toughness.

It has been proposed to use hybrid matrix thermosetting resins includinga high molecular weight thermoplastic polymer, as a particulatedispersion as disclosed for example in GB-A-2060490, or as a particulatecoating or film interleave of the fiber-reinforced matrix resin prepregsas disclosed in U.S. Pat. No. 5,057,353. Nevertheless dispersion istypically poor due to difficulty in controlling distribution ofparticles and uniformity of particle size which can influence rate anddegree of melting, and the barrier effect of a continuous film duringresin infusion.

U.S. Pat. No. 5,288,547 discloses prepregs for curable compositionscomprising a porous thermoplastic polymer membrane interleave. Themembrane is laid up against a sheet of reinforcing fiber and melting atelevated temperature and pressure to impregnate the fibers;alternatively prepreg is laid up with membrane between and melted toimpregnate prior to curing to form a composite part; alternatively themembrane proposed for RTM application is laid up between layers of dryfiber in a mold, melted to impregnate, and liquid resin injected intothe mold.

While these technologies go some way to alleviating the problemsassociated with thermoplastic toughened resin infusion systems, there isstill a need for a more versatile solution with more flexibility andcontrol of nature and amount of toughening agent and increasedperformance properties. Specifically, the need for an improved means ofincorporating and controlling the amount of thermoplastic toughener in aresin infusion system remains unsatisfied. Indeed, there remains a needto introduce greater amounts of thermoplastic polymers for tougheninginto the system.

SUMMARY OF THE INVENTION

The present invention surprisingly overcomes the problems of priorprepreg and resin infusion systems. A preferred embodiment of thepresent invention comprises a non-woven veil of resin-solublethermoplastic interposed between plies of dry structural reinforcementfiber for resin infusion to enhance inter-laminar toughness and otherproperties such as flame, smoke and toxicity (FST), and preformingcapabilities.

In the broadest aspect of the present invention there is provided aresin-soluble thermoplastic element for a curable composition adapted tobe interposed between layers of structural reinforcement fibers whereinthe resin-soluble thermoplastic polymer is in a solid phase and adaptedto undergo at least partial phase transition to a fluid phase on contactwith a component of the curable composition in which it is soluble, at atemperature which is less than the temperature for substantial onset ofgelling and/or curing of the curable composition and which temperatureis less than the inherent melting temperature of the resin-solublethermoplastic element.

The present invention relates to a resin-soluble thermoplastic elementfor use in a curable composition wherein the element is adapted todissolve in the curable composition; a method for the preparationthereof; a support structure or carrier for a curable compositioncomprising the at least one resin-soluble thermoplastic element togetherwith reinforcing fibers, configurations of support structures andcarriers, methods for preparation thereof, a curable compositioncomprising the at least one resin-soluble thermoplastic element or thesupport structure or carrier and a curable resin matrix; a kit of partscomprising the components thereof and a method for selection thereof, amethod for preparation and curing thereof; and a cured composite orresin body obtained thereby, and known and novel uses thereof.

The present invention is exceptionally suited for resin infusion systemsin that it provides the thermoplastic toughening component in virtuallyany amount desired through the placement of the thermoplastic elementonto the dry fibers, or interposed between layers thereof, withadditional thermoplastic toughening material not required within theepoxy resin being infused.

Furthermore the permeable nature of the resin-soluble thermoplastic veilof the present invention provides more uniform flow and delivery ofmatrix resin throughout the preform than other thermoplastic films,which are not permeable.

Furthermore, the present invention utilizes thermoplastic polymers thatare soluble within a resin system, thus allowing for more uniformdistribution into the resin matrix than insoluble, meltablethermoplastics. Additionally, as the thermoplastic polymer of thepresent invention does not require high temperatures to melt in order todiffuse into the resin matrix, high processing temperatures are notrequired and out of autoclave manufacturing is possible.

A preferred embodiment of the present invention is a non-woven veilcomprising a polyaromatic resin-soluble thermoplastic fiber compatiblewith epoxy resin matrices.

A preferred embodiment of the present invention provides a non-wovenveil of resin-soluble thermoplastic adapted to incorporate modifierssuch as additives, curing agents, metal flakes and powders, hard andsoft particles, fire retardants, nanoparticles, etc, that cannot bereadily incorporated in a standard resin.

A preferred embodiment of the present invention comprises a curableresin matrix and a resin-soluble non-woven thermoplastic veil fortoughening, together with optional additional toughening agents,reinforcing fibers, catalysts, curing agents, additives, fillers and thelike.

A further aspect of the present invention is its ability to providelocalized toughening of a composite. The present invention has theability to lay up additional layers of thermoplastic toughening fabricin the areas requiring additional toughening.

A further aspect of the present invention is its ability to providestabilization to a preform structure. A preferred embodiment interposesthe resin-soluble thermoplastic veil between adjacent plies of, and incontacting relation to, structural reinforcement fibers in a preform;heating the preform sufficiently to soften the resin-solublethermoplastic veil fibers in contact with the structural reinforcementfibers; allowing the thermoplastic fibers to at least partially adhereto the structural reinforcement fibers; and cooling the preform tosolidify and harden the thermoplastic fibers to stabilize.

A further aspect of the present invention is that the resin-solublethermoplastic polymer element may be present as a film in the form of aninterleaf, with resin matrix film, or as a permeable or foamed filmimpregnated with resin matrix or the like.

A preferred embodiment of the present invention includes a method ofmanufacturing a non-woven resin-soluble thermoplastic veil comprisingthe steps of heat blow extruding a resin-soluble thermoplastic polymerinto a plurality of fibers onto a mandrel and cooling the resultingrandom veil. Alternatively, the fibers may be chopped into a pluralityof thermoplastic pieces of lengths from about 0.01 inches to about 15inches; the pieces scattered onto a mandrel heated to between about 100°C. and 370° C., and cooling the veil.

A further preferred embodiment of the present invention involves atoughening fabric for composites comprising a plurality of resin-solublethermoplastic threads randomly adhered to one another into a porousfabric sheet.

A further preferred embodiment of the present invention involves acomposite manufactured using a resin-soluble thermoplastic non-wovenveil.

The present invention is further characterized as capable of being madefrom continuous, staple or chopped fibers.

The present invention is further characterized as capable of being madein the form of a fiber, film, non-woven mat or veil or the like.

The resin-soluble thermoplastic polymer element is preferablycharacterized as capable of being drawn from a laminating type machinewithout damaging the product.

The resin-soluble thermoplastic polymer of the present invention ischaracterized as soluble in any thermoset resin (vinylester, polyester,phenolic, epoxy, bismaleimides, cyanate esters), preferably soluble inepoxy and cyanate esters.

A further aspect of the present invention is that it is adapted for usewith a curable resin matrix that includes at least one thermoplasticpolymer, the curable resin matrix providing elevated levels ofthermoplastic polymer wherein the thermoplastic polymer is present in afirst amount in fluid phase as a curable resin matrix component andadditionally is present in a second amount in the form of at least oneresin-soluble thermoplastic polymer element in solid phase.

The present invention surprisingly finds that it is possible to provideelevated levels of a thermoplastic polymer adaptable to a resin infusionsystem, or the like by providing a part thereof as a resin-solublethermoplastic polymer element in the form of a fiber, non-woven veil,mat, film, or the like capable of dissolving in a curable resin matrix,whereby it may be controllably combined in a curable resin matrix bymeans of at least partial phase transition as herein before defined toprovide a polymer blend having desired properties.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates the concentration gradient of the toughening elementof the present invention interposed between plies of structuralreinforcement fibers after diffusing into the curable resin that may becontrolled to provide a higher concentration of toughening materialwhere it is most preferred, between the structural reinforcement fiberplies.

FIG. 2 illustrates data supporting the improved toughness of a compositemanufactured using the present resin-soluble thermoplastic veil for aresin infused composite part.

FIG. 3 illustrates the use of the present invention for localizedtoughening.

FIG. 4 illustrates the manufacturing of a composite incorporating theresin-soluble thermoplastic veil of the present invention with resininfusion technology.

FIG. 5 illustrates the detailed lay up of the resin-solublethermoplastic veil of the present invention interposed betweenstructural reinforcement fiber plies.

FIG. 6 illustrates the process for stabilizing a structuralreinforcement fiber ply into a stable preform using the resin-solublethermoplastic veil of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

By the term resin infusion is meant the method of introducing orinjecting a curable resin matrix into a reinforcement fiber substrate,structural fabric or preform for composite part manufacture.

By the term resin-soluble thermoplastic polymer element, thermoplasticfiber, or polymer fiber is meant any flexible polymer element which isboth chemically and physically adapted to at least partially dissolve ina curable resin matrix making up the curable composition, at a firsttemperature, that is less than the melt temperature of the resin-solublethermoplastic polymer, wherein the resin-soluble thermoplastic polymeris at least partially dispersed into the curable resin matrix as acommon phase by dissolution whereby it loses, at least partially, itsphysical element form to the curable composition, and whereby theresin-soluble thermoplastic reacts with the curable resin matrix at asecond, chain linking, temperature to cross-link with the curable resinmatrix. These thermoplastic fibers may be adapted to a non-woven veil ormat.

By the term method of manufacturing is meant a means for forming anon-woven veil by the arrangement of chopped or continuous fibers into asheet, web, veil, mat or the like. The fibers can be chopped fibers,staple fibers packed in bales or continuous filaments extruded frommolten polymer granules or pellets.

By the term flow temperature is meant the temperature at which theresin-soluble thermoplastic polymer and curable resin matrix attain asuitably fluid state to enable a degree of polymer chain mobility.Preferably the flow temperature corresponds to a solution temperature atwhich the resin-soluble thermoplastic polymer dissolves and whichtemperature is less than a chain linking temperature.

By the term chain linking temperature is meant a temperature at which aresin-soluble thermoplastic polymer chain linking reaction may beinitiated. Preferably the chain linking temperature is higher than theresin-soluble thermoplastic polymer dissolution temperature. Preferablythe chain linking temperature corresponds to the gelling or coalescencetemperature of the curable matrix resin.

By the term reinforcing fibers is meant structural or stabilizing fibersthat are insoluble fibers as known in the art which stiffen composites,such as organic or inorganic polymer, carbon, glass, quartz, aramid,PBO, polyethylene, inorganic oxide, carbide, ceramic, boron or metal andthe like fibers. Poly {2,6-diimidazol [4,5-b: 4′s′-e]pyridinylene-1,4(2,5-dihydroxy) phenylene} (PIPD commercially availablefrom AkZO Nobel Central Research, The Netherlands), graphite and basalt.

By the term preform is meant to be any number of physical presentationssuch as in the form of a support structure or preform as known in theart.

By the term epoxy resin matrix is meant the curable resin matrix forresin infusion systems that is introduced by injection, infusion or thelike into a preform to manufacture a composite part.

The present invention is a resin-soluble thermoplastic element for acurable composition adapted to be interposed between layers ofreinforcement fibers wherein the resin-soluble thermoplastic element isin a solid phase and adapted to undergo at least partial phasetransition to a fluid phase on contact with a component of the curablecomposition in which it is soluble, at a temperature which is less thanthe temperature for substantial onset of gelling and/or coalescence ofthe curable composition and which temperature is less than the inherentmelting temperature of the resin-soluble thermoplastic element.

The resin-soluble thermoplastic element for a curable composition ispreferably in the form of a veil. The resin-soluble thermoplastic veilshall have a preferred areal weight of about 2 to about 150 grams persquare meter (gsm), and most preferably between about 5 and about 50gsm. The resin soluble thermoplastic veil shall have a preferred realweight range of ±20% on nominal and most preferably of ±10% on nominalreal weight.

The permeability of the resin-soluble thermoplastic veil of the presentinvention provides more uniform flow and delivery of curable resinmatrix throughout a preform than traditional thermoplastic films whichare not porous.

The resin-soluble polymer may be present in the form of an interleaffilm.

The veil of the present invention is preferably comprised of a randommat of continuous or chopped polymer fibers that have a diameter ofabout 0.1 to about 100 microns and preferably between about 1.0 andabout 50 microns.

The resin-soluble thermoplastic polymer of the present invention isselected from the group consisting of rubber, elastomeric polymers,thermoplastic polymers and combinations thereof. Preferably, the polymeris a thermoplastic polymer and more preferably it is an amorphousthermoplastic polymer or a crystalline polymer having a relatively lowmelting point, but generally in excess of about 300° F.

More particularly, the resin-soluble thermoplastic polymer according tothe present invention can be any type of resin-soluble fibrousthermoplastic toughening material such as cellulose derivatives,polyester, polyamide, polimide, polycarbonate, polyurethane, poly(methylmethacrylate), polystyrene, polyaromatics; polyesteramide,polyamideimide, polyetherimide, polyaramide, polyarylate, polyacrylate,poly(ester) carbonate, poly(methyl methacrylate/butyl acrylate),polysulphone, polyethersulphone, polyetherethersulphonepolyethersulphone-etherketone, and copolymers and combinations thereof.Preferably the thermoplastic toughening material is a polyethersulphoneor polyetherethersulphone polyethersulphone-etherketone.

More preferably, the resin-soluble thermoplastic polymer comprises apolymer preferably having a number molecular weight from about 1000 toabout 60,000, more preferably in the range of from about 2,000 to about20,000.

Examples of suitable thermoplastic polymers include, but are not limitedto members of the group consisting of cellulose derivatives, polyester,polyamide, polyimide, polycarbonate, polyurethane, poly(methylmethacrylate), polystyrene, polyaromatics; polyesteramide,polyamideimide, polyetherimide, polyaramide, polyarylate, polyacrylat,poly(ester) carbonate, poly(methyl methacrylate/butyl acrylate),polysulphone, polyethersulphone, polyetherethersulphonepolyethersulphone-etherketone, and copolymers and combinations thereof.

A particularly preferred polyaromatic thermoplastic polymer for use asthe fiber in the present invention preform is a polyaromatic sulphonecomprised of ether-linked repeating units or thioether-linked repeatingunits, the units being selected from the group consisting of—(PhAPh)_(n)—and—(Ph)_(a)—

-   -   wherein A═CO or SO₂, Ph is phenylene, n=1 to 2 and can be        fractional, a=1 to 4 and can be fractional, with the proviso        that when a exceeds 1, the phenylenes are linked linearly        through a single chemical bond or a divalent group other than        —CO— or —SO₂— or are fused together directly or via a cyclic        moiety selected from the group consisting of an acid alkyl        group, a (hetero) aromatic, a cyclic ketone, a cyclic amide, an        imide, a cyclic imine and combinations thereof. Preferably, the        polyaromatic sulphone comprises polyethersulphone (PES) and more        preferably, a combination of polyethersulphone-etherketone and        of polyether ether sulphone linked repeating units (PES:PEES),        in which the phenylene group is meta- or para-; wherein the        phenylenes are linked linearly through a single chemical bond or        a divalent group other than sulphone, or are fused together. The        preferred relative proportions of the repeating units of the        polyarylsulphone in the PEES:PES may be expressed in terms of        the weight percent SO₂ content defined as 100 times (weight of        S0₂)/(weight of average repeat unit). The preferred SO₂ content        is at least about 35, more preferably at least about 30 and most        preferably at least about 22%. When a=1 this corresponds to        PES/PEES ratio of at least 20:80, preferably in the range 25:75        to 75:25 and most preferably 35:65 to 65:35.

Alternatively, when the thermoplastic polymer for use as the fiber ispolyurethane, it is more preferably a thermoplastic polyurethane rubberand when the fiber is a polyacrylate, the polyacrylate is at least 85 wt% acrylonitrile. When the thermoplastic polymer is a polyamide, it ispreferably a nylon material and more preferably an amorphous nylon.

The polymer may be characterized by a range of Molecular Weight (MW)which may typically be defined either by number average molecular weight(Mn), peak MW and other means, usually determined by NMR and GPC.Preferably the polymer is selected in the range up to 70,000 for example9000-60,000 for toughening, and in this case the Mn of the polyaromaticis suitably in the range 2000 to 25000, preferably 2000 to 20000, morepreferably 5000 or 7000 to 18000, most preferably 5000 or 7000 to 15000.

The polyaromatic is preferably of relatively low molecular weight. Italso preferably contains in-chain, pendant or chain-terminating chemicalgroups which are capable of self-assembling to form higher molecularweight complexes through non covalent bonds with similar or differentchemical groupings in the polymer. These maybe, for example, hydrogenbonds, London forces, charge transfer complexes, ionic links or otherphysical bonds. Preferably the non-covalent bonds are hydrogen bonds orLondon forces which will dissociate in solution to regenerate therelatively low molecular weight precursor polyaromatic. The polyaromaticpreferably contains pendant or chain-terminating groups that willchemically react with groups in the thermosetting resin composition toform covalent bonds. Such groups may be obtained by a reaction ofmonomers or by subsequent conversion of product polymer prior to orsubsequently to isolation. Preferably groups are of formula:—A′—Y

-   -   where A′ is a divalent hydrocarbon group, preferably aromatic,        and Y is a group reactive with epoxide groups or with curing        agent or with like groups on other polymer molecules.

Examples of Y are groups providing active hydrogen especially OH, NH₂,NHR′ or —SH, where R′ is a hydrocarbon group containing up to eight (8)carbon atoms, or providing other cross-linking reactivity especiallyepoxy, (meth)acrylate, cyanate, isocyanate, acetylene or ethylene, as invinyl allyl or maleimide, anhydride, oxazaline and monomers containingsaturation. Preferred end groups include amine and hydroxyl.

A particular advantage is that the polymer element may have lowmolecular weight, but be adapted to react on curing to provide thehigher molecular weight required for effective toughening or the like.Specifically the polymer may comprise chains of at least one aromaticpolymer or a mixture thereof together with at least one chain linkingcomponent wherein the at least one aromatic polymer comprises polymerchains in a first range of 2000 to 11000 Mn, especially 3000 to 9000 Mnand characterized by a polymer flow temperature, and wherein one of theat least one polyaromatic and the at least one chain linking componentcomprises at least one reactive end group and the other comprises, atleast two linking sites reactive end groups Y and chain linking sites, Zare selected OH, NH₂, NHR or SH wherein R is a hydrocarbon groupcontaining up to eight (8) carbon atoms, epoxy, (meth)acrylate,(iso)cyanate, isocyanate ester, acetylene or ethylene as in vinyl orallyl, maleimide, anhydride, acid, oxazoline and monomers containingunsaturation characterized in that a plurality of the end groups areadapted to react with the linking sites at chain linking temperature inexcess of the polymer flow temperature to form linked polymer chains ina second range of 9000 to 60000 Mn, especially 11000 to 25000 Mn, whichis in excess of the first range, substantially thermoplastic in nature.

The fiber characteristics include, but are not limited to yarns ormonofilaments of spun strands, extruded strands, cast strands,continuous fibers, random fibers, staple fibers, discontinuous fibers,chopped fibers, whiskers, filaments, ribbons, tapes, hollow fibers,veils, fleeces, and combinations thereof.

The fiber may be both a yarn made up of multiple monofilaments or singleand multiple monofilaments. Preferably the fiber (or yarn) comprisesfibers each having a diameter less than or equal to about 100 micron.Preferably a fiber or filament has a diameter d, or is a film or ribbonhaving thickness t wherein d or t are in the range up to 100 micron,preferably from about 1.0 to about 80.0 micron, more preferably betweenabout 10 and about 50 micron.

The thermoplastic veil should be provided in dimensions of about 1 inchto about 200 inches wide and preferably between about 20 inches andabout 70 inches.

The fiber cross section can be a variety of different shapes to allowcontrol of the wetting of the polymer fiber such as star, square,cauliflower or kidney bean shape as well as standard oval, circular andflat ribbons. Additionally, the fiber may be hollow or contain differentmaterials in different concentric layers: for example, an inner core andan external shell. Preferably, the fibers are cylindrical in shape witha circular cross section for optimal surface area for dissolution.

The thermoplastic veil preferably comprises chopped fibers with anaverage length of about 10 microns to about 100 inches and preferablyabout 1 inch to about 20 inches.

The thermoplastic veil should at least partially maintain its solidphase while resin is infused into the system so as to not dissolve intothe resin and increase the resin viscosity during infusion. Dissolutionduring resin infusion would impede the resin flow through thereinforcement fibers. More preferably the veil should not significantlyor appreciably dissolve during resin infusion temperatures between 20°C. and about 90° C. or preferable resin infusion temperatures of betweenabout 30° C. and about 65° C.

The thermoplastic veil should begin appreciable dissolution into thecurable resin matrix above 90° C. with substantially completedissolution occurring between about 90° C. to 175° C. with the preferreddissolution temperature being between about 120° C. and 140° C. and mostpreferably about 130° C.

The melting temperature of the resin-soluble thermoplastic element ofthe present invention is from about 175° C. to about 400° C. or greaterand preferably between about 250° C. and 400° C. The particular meltingtemperature of the resin-soluble thermoplastic element is dependent uponthe molecular weight of the particular thermoplastic employed.

The thermoplastic veil does not substantially react with the curableresin matrix system at the dissolution temperature.

The thermoplastic veil may further comprise a combination of more thanone resin-soluble thermoplastic polymer in first or second amounts.

The methods of manufacturing the thermoplastic veil of the presentinvention include drylaying; airlaying;meltblowing/spunlaying/spunbonding; and wetlaying. Preferably theresin-soluble thermoplastic veil of the present invention ismanufactured through meltblowing.

In the meltblowing manufacturing process, the resin-solublethermoplastic polymer in powder form is melted at between about 250° C.and 400° C., preferably about 325° C. and extruded through a pluralityof spinnerets. The orifice size of the spinneret is between about 0.1microns to about 100 microns and preferably between about 1.0 micronsand about 50 microns. Air in the temperature range of 350° C. to 500° C.is blown onto the fibers extruded onto a conveyor from the spinnerets torandomize the fibers into a veil.

The meltblowing process can use polymer powder or granules which aremelted into a molten polymer that is extruded through the spinnerets.The continuous filaments are deposited onto a conveyer and cooled toform a uniform non-woven veil. Some remaining heat content of the softfilaments can cause the filaments to at least partially adhere to oneanother, but this cannot be regarded as the principal method of bondingthe filaments to one another. This process has the advantage of givingthe resulting non-woven veil a greater tensile strength and resistanceto tearing or fraying during manufacture of the veil and duringmanufacture of a composite.

Co-extrusion of second thermoplastic polymer component is possible,usually to provide additional toughening properties, fire retardancyproperties, bonding capabilities and the like.

The drylaying process is an alternate means of manufacturing theresin-soluble thermoplastic veil of the present invention and startswith the opening of bales of thermoplastic polymer fibers which areblended and conveyed to the next stage by air transport. The fibers arethen combed into a veil by a carding machine, which is a rotating drumor series of drums covered in fine wires or teeth. The preciseconfiguration of cards will depend on the fabric weight and fiberorientation required. The veil can be parallel-laid, where most of thefibers are laid in the machine direction, or they can be random-laid.Typical parallel-laid carded veils result in good tensile strength, lowelongation and low tear strength in the machine direction and thereverse in cross direction. Relative speeds and veil composition can bevaried to produce a wide range of properties.

The airlaying process is another method of drylaying. In airlaying, thefibers, which can be very short, are fed into an air stream and fromthere to a moving belt or perforated drum, where they can form arandomly oriented veil. Compared with carded veils, airlaid veils have alower density, a greater softness and an absence of laminar structure.Airlaid veils offer great versatility in terms of the fiber blends thatcan be used.

In the wetlaying process, dilute slurry of water and fibers is depositedon a moving wire screen and drained to form a veil. The veil is furtherdewatered, consolidated, by pressing between rollers and dried. Wetlaidveil-forming allows a wide range of fiber orientations ranging from nearrandom to near parallel. The strength of the random oriented veil issimilar in all directions in the plane of the fabric.

A further method of manufacturing a non-woven resin-solublethermoplastic fabric veil comprises the steps of heat blow extruding ofthe resin-soluble thermoplastic into a thread; chopping the thread intoa plurality of thermoplastic pieces of lengths from about 10 microns toabout 100 inches and preferably about 1 inch to about 20 inches;scattering the pieces onto a heated mandrel and cooling the veil.

Preferably polymers in the form of fibers or film are obtained bycontinuous extrusion of resin melt onto reels and film forming orspinning as known in the art of synthetic textiles manufacture bymechanical stretching with heating, more preferably by providing thepolymer melt, drawing off in elemental shape, subjecting to a heatingand mechanical stretching regime which may orient polymer chains andrender the element elastomeric and predisposed to dissolution, andcooling, preferably by pulling in air for a desired distance, e.g. 50 to500 mm. Preferably polymer melt is drawn off through a die head or thelike providing a desired number of apertures or slots, using a pump withcontrolled pump rate for a desired linear density (tex) of polymer forexample up to 180 tex.

The thermoplastic polymers may be prepared from micronized orunmicronized polymer, pellets, particles or other extrudate and thelike. Preferably fibers are prepared as multifilaments of up to 20 sameor different polymer filaments, which are drawn off from the moltenpolymer, cooled and optionally twisted as desired, and then subjected toheating and stretching. The multifilament is more resistant to breaking;there is a trade off between higher strength and lower flexibility inselection of filaments and twists/meter.

In a further aspect of the present invention there is provided a processfor the preparation of a curable composition as known in the artcomprising contacting a resin-soluble thermoplastic veil with a curableresin matrix for example by interleaving, impregnating, injecting orinfusing, mixing and the like.

The composition may then be laid up with other component parts such asreinforcing fibers to provide the curable composition, or othercomposite parts such as metal or polymer or other bodies or structuresprior to curing in known manner.

Another aspect of the present invention is for use as interleaves forintroducing thermoplastics into the interlaminar region of conventionalprepregs. The resin-soluble thermoplastic veils can also be utilized indry preforms where the open weave structure optimally allows for theinjection/infusion of the thermosetting curable resin matrix to occurthroughout the preform. This is unlike the inclusion of continuous filmswhich act as obstructions to the resin flow which in turn can lead toporosity and poor mechanical and environmental performance.

Another aspect of the present invention is for stabilization of performsand perform precursors. The present invention provides for the use of aresin-soluble thermoplastic veil to combine, bond, stabilize, debulk andpreform structural fibers, fabrics, textiles and preforms by using acombination of time, temperature and pressure.

For the purposes of the present invention, the terms combine, bond,stabilize, debulk and preform are used to mean 1) the stabilization offibers or single sheet, layer or ply or multiple sheets, layers or pliesof structural fabric so that it can be moved, cut, transported, resininfused, or handled in a typical manner without fraying, unravelling,pulling apart, bending, wrinkling or otherwise distorting the integrityof the structural fabric, 2) the stabilization and binding together ofmultiple layers of reinforcing or structural fabrics for cutting,molding or shaping, by placing in a mold or otherwise so that theresulting preform will not be distorted by being moved, transported ormanipulated in any way and so that the fibers that make up thestructural fabrics remain intact during resin infusion, and 3) fixing apreform in a desired shape.

The stabilized preform according to the present invention comprises aveil according to the present invention comprising all of itscharacteristics and features, that has been subjected to a stabilizingtemperature that is preferably suitable to soften the stabilizing fiberand is most preferably of from about 100° C. to about 250° C. for a timeperiod from about 5 seconds to about 100 minutes or more.

The stabilized preform according to the present invention can typicallycomprise more than one layer of structural fabric. The layers or pliesof structural fabric can be stacked, cut and shaped prior to having beensubjected to a stabilizing temperature.

In the present process the heat can be applied in any manner and fromany source, preferred examples including, but not being limited to heatsources selected from infrared, microwave, convection, induction,ultrasonic, radiant and combinations thereof. The heat should preferablybe applied in an amount that is sufficient to soften the stabilizingfiber. More preferably the heat is applied at a temperature of fromabout 125° C. to about 185° C. for a time period of from about 1 min toabout 100 min. The step of applying heat can be performed under a vacuumof from about 500 mbar to about 999 mbar or under pressure applied byusing a press, nip rollers and the like.

Optionally, in the present process more than one layer of structuralfabric can be provided. The process can then further comprise the stepsof stacking and cutting the layers or plies of structural fabric afterthe step of integrating the stabilizing fiber into the structural fabricand before the step of applying heat. The present process can optionallyfurther comprise the step of shaping the layers of structural fabricafter the layers have been stacked and cut and before the step ofapplying heat

In the present process, the step of integrating the resin-soluble veilinto the structural fabric can be carried out by using any known methodfor integrating or incorporating a veil into a structural fabric,preferred examples of which include a method selected from punching,overwinding, intermeshing, aligning, hot compacting and combinationsthereof.

The present invention also comprises a process for debulking a preformprecursor comprised of reinforcing fibers for composite materialmanufacture. The process comprises the same steps utilized forstabilization in order to reduce the thickness of the stack. The step ofapplying heat is preferably performed under a vacuum of from about 500mbar to about 999 mbar or under pressure applied by using a press, niprollers and the like.

The process can further comprise the step of shaping the stack ofstructural fabric after the step of cutting and before the step ofapplying heat.

The resin-soluble thermoplastic fibers of the present invention and anyreinforcing fibers are incorporated with the curable resin matrix at anysuitable stage in the process.

The resin-soluble thermoplastic veil of the present invention can alsocarry modifiers such as additional toughening agents, additives, curingagents, metal flakes, fire retardants, nanoparticles, etc, that cannotbe included in a standard resin matrix or adhesive.

Localized toughening of a composite is also uniquely possible throughthe present invention by providing a means for separately laying upadditional layers of thermoplastic toughening fabric in the areasrequiring additional toughening.

The resin-soluble thermoplastic veil is preferably capable of beingcombined with structural reinforcement fibers by being drawn from alaminating type machine without damaging the product.

The resin-soluble thermoplastic veil of the present invention ischaracterized as being compatible and complementary to a curable resinmatrix system. In this regard the resin matrix may further comprise acompatible or complementary thermoplastic toughening material that isthe same or different than the resin-soluble thermoplastic in the veil.The thermoplastic incorporated in the resin may be soluble or insoluble.

The resin-soluble thermoplastic polymer of the present invention ischaracterized as being soluble within the curable resin matrix. Thepolymer should have the ability to at least partially and preferablycompletely dissolve, (as opposed to melt) in the curable resin matrix,most preferably a curable resin matrix for infusing into a preformcomprising reinforcement fibers and the present invention to manufacturethe composite. The dissolution characteristic thus allows for morecontrolled distribution into the resin matrix than meltablethermoplastics.

Additionally, as manufacturing with the thermoplastic polymer does notrequire a high temperature to dissolve and diffuse the thermoplasticpolymer into the resin matrix, high processing temperatures are notrequired and out of autoclave manufacturing is possible.

Dissolution should preferably occur at a dissolution temperature belowthat of the cure temperature for the resin and below the melttemperature of the polymer.

The polymer element is preferably adapted to dissolve during thepreliminary stages of the process, after resin infusion, duringtemperature ramping to the temperature for onset of gelling coalescenceand/or curing.

The polymer element is substantially independently undetectable in theproperties of the cured composition. It is a particular advantage thatthe polymer element is soluble and may be physically traceless as aseparate phase in the cured composition.

The polymer element preferably chain links with the curable resin matrixand contributes to the properties of the resulting cured composite.

In a preferred embodiment of the present invention, the fluid phase ofthe polymer element undergoes excellent dispersion by solvating effectin the curable component. This is particularly important to theproperties of the resulting cured composite product. Raman spectroscopyat co-ordinates throughout the cured composite product shows completedispersion, with identical scans at each coordinate.

Thus, through control of the cure cycle, the extent of dispersion of thethermoplastic polymer in the curable resin matrix may be controlled toallow for full dispersion or local concentration of the polymer.

Phase separation in the case of a fiber as a polymer element ischaracterized by complete dissolution. Phase transition e.g. solution ofpolymer element may be determined or monitored with use of any suitabletechniques, for example TEM, SEM, neutron scattering and the like andsuch techniques may be employed by those skilled in the art to determinesuitable polymer element characteristics and curable resin matrixcharacteristics and processing conditions for commercial production ofcured compositions.

The polymer is preferably adapted to undergo phase transition, i.e. toat least partially dissolve in the resin matrix at a temperature Ts in arange at least part of which is less than the cure temperature of theresin matrix Tc. The polymer element may be configured in manner toimprove or hinder thermal conductivity and speed or slow transfer ofheat into the element to endure rapid or delayed solution thereof.

The polymer element may undergo complete or partial phase transition,e.g. may completely dissolve, or may partially dissolve whereby aportion thereof is dispersed into the matrix and a portion retains itselemental form, either by ensuring that precuring time and temperatureare insufficient for complete dissolution or preferably by providing thepolymer as a blend or co-polymer with one or more further insolublepolymers, for example in the form of a random or block co-polymer orother blend or derivative with organic or inorganic substrates. By thismeans the polymer element may be combined with one or more furtherpolymers or other soluble or insoluble organic or inorganic substratesin the cured composition.

The desired level resulting cured composite toughness is obtained bycontrol of the morphology and phase sizes in the thermoset/thermoplasticblend through the chemistries of the thermoplastic polymer and thethermosetting resin precursors, as well as the other parameters of anydesired morphology.

Composites formed through the use of the instant resin-solublethermoplastic veil in a resin infusion process exhibit equivalenttoughness to standard prepreg systems formed with equivalent amounts ofthermoplastic polymer toughening material.

The percentage of thermoplastic polymer dissolved into the resin matrixbetween the structural reinforcement fibers provides surprising benefitsover toughening material woven into the structural reinforcement fibersor pre-dissolved into the curable resin matrix. This is because whilethe total amount of thermoplastic toughening material in the system maybe the same as a standard prepreg system, the concentration throughoutthe finished composite may be controlled, and in particular betweenplies. In the present invention, the dispersion of the thermoplasticpolymer may be controlled such that the concentration of thermoplastictoughening material may be highest between the structural reinforcementfiber plies, decreasing in concentration as you approach a structuralreinforcement fiber ply, being nominal within the fiber ply.

FIG. 1 illustrates a possible polymer toughening material concentrationgradient. This toughening material concentration gradient can providefor the increased toughening between fiber plies, rather than within thestructural reinforcement fiber ply.

FIG. 2 shows comparison data for toughness tests among a system with thepolymer toughening material interleafed between layers of reinforcementfibers and without. This illustrates the improved toughening capabilityof the present invention.

Localized toughening is also available through the use of the presentinvention in the instance where fasteners, openings or other forms ofpenetrations through the finished composite are required. A weakness inthe composite in the form of cracking due to stresses around thepenetration is created. It is known in the art that that this weaknessis most prominent between the first and second ply, known as first-plyfailure. The present invention can provide localized toughening in thesefailure prone regions, and in particular between the first and secondply through the addition of more layers of non-woven tougheningmaterial. This additional toughening material reduces cracking andweakness due to the penetrations.

The concentration profile of dissolved toughening material as discussedabove is only increased where additional toughener is added.

FIG. 3 illustrates this localized toughening.

It is further intuitive that this form of localized toughening may beincorporated anywhere with the composite and indeed may be incorporatedthroughout the composite at a local stress site.

The present invention is further adaptable to chain linking during thecure cycle to further toughen the resulting cured composite. Chainlinking components are preferably selected from the formulaB(Z-)_(n)wherein B is an oligomer or polymer backbone or is an aliphatic,alicyclic or aromatic hydrocarbon having from 1 to 10 carbon atoms andoptionally including heteroatoms N, S, O and the like and optionallysubstituted, or is C, O, S, N or a transition metal nucleus or is asingle bond, n is a whole number integer selected from 2 to 10000preferably 2 to 8 or 5 to 500 or 500 to 10000.

Accordingly it will be apparent that self reaction between methacrylateended polymer and chain linking component or between maleimide endedpolymer and chain linking component or between oxazoline ended polymerand chain linking component for example is possible and within the scopeof the present invention.

In a preferred embodiment, the reactive end group is hydroxy andcorresponds to a linking site functionality which is epoxy, wherebyreaction thereof produces a hydroxy ether linkage in polymers ofincreased number average molecular weight having either hydroxy or epoxyend groups as desired. Alternatively, the reactive end group is NH₂ andthe linking site functionality is anhydride, whereby reaction thereofproduces an imide linkage in polymers of increased number averagemolecular weight having NH₂ or anhydride end groups. Alternatively thereactive end group is NH₂ and the linking site functionality ismaleimide. Mixtures of the above may be employed to produce a mixedarchitecture including a plurality of reactive end group-linking sitecombinations.

Preferred linking components include multifunctional epoxy resins,amines and in particular triazines, and anhydrides. Suitable epoxyresins and amines are selected from resins hereinafter defined formatrix resins, and are preferably selected from MY0510, Epikote 828[O(CH₂CH) CH₂OPh]₂C(CH₃)₂ and the Cymel class of epoxies including Cymel0510, benzophenone tetra carboxylic acid dianhydride (BTDA)[O(CO)₂Ph]₂CO, and maleic anhydride.

Preferably polymer elements comprising two or more polymers comprise ablend or copolymer of amorphous polymers or of amorphous and semicrystalline polymer. This is of particular advantage in enabling thepreparation of multiblock compositions having lowered processingtemperatures whilst nevertheless retaining excellent product propertiessuch as solvent resistance.

The present invention is particularly beneficial to preforms for resininfusion technology. In a further aspect of the invention there isprovided a resin-soluble thermoplastic veil for interposing betweenstructural fiber layers for a curable composition comprising at leastone resin-soluble polymer element together with structural elements,preferably reinforcing fibers, wherein the at least one resin-solublepolymer element is present in solid phase and adapted to undergo atleast partial phase transition to fluid phase on contact with a resinmatrix component of a curable composition in which the element issoluble, at a temperature which is less than the temperature forsubstantial onset of gelling and/or curing of the curable component.

The structural reinforcement fibers can be of any type of textilestructure known in the art for manufacturing composite materials madefrom reinforcing structural fabrics adaptable to infused liquid resins.Examples of suitable fabric types or configurations include, but are notlimited to: all woven fabrics, examples of which include, but are notlimited to polar weaves, spiral weaves and uniweaves; all multiaxialfabrics, examples of which include, but are not limited to multi warpknitted fabrics, non crimp fabrics (NCF) and multidirectional fabrics;knitted fabrics braided fabrics; tailored fiber placement fabrics suchas for example only fiber placement and embroidered fabrics, allnon-woven fabrics, examples of which include but are not limited to matfabric, felts, veils and chopped strands mats and fabrics that arecomprised of combinations thereof.

The fibers that make up the reinforcing structural fabric can be anytype of fiber known in the art of composites, examples of which include,but are not limited to spun strands, extruded strands, cast strands,continuous fibers, random fibers, discontinuous fibers, chopped fibers,whiskers, filaments, ribbons, tapes, hollow fibers and combinationsthereof. Suitable materials from which the fibers can be made include,but are not limited to those selected from the group consisting ofcarbon, aramid, quartz, boron, glass, polyethylene, polybenzazole,poly(p-phenylene-2,6-benzobisoxazole) polybenzothiazole alumina,zirconia, silicon carbide, Poly {2,6-diimidazol [4,5-b: 4′s′-e]pyridinylene-1, 4 (2,5-dihydroxy) phenylene} (PIPD commerciallyavailable from AkZO Nobel Central Research, The Netherlands), graphiteand basalt.

-   -   and combinations thereof.

The structural reinforcement fiber layers used in conjunction with thepresent invention may be prepared in continuous manner for example as aroll of fabric which may be tailored by stitching and weaving in desiredmanner.

Structural reinforcement fibers as hereinbefore defined can be short orchopped typically of mean fiber length not more than 2 cm, for exampleabout 6 mm. Alternatively, and preferably, the fibers are continuous andmay, for example, be unidirectionally-disposed fibers or a woven fabric,i.e. the composite material of structural reinforcement fibers and resinmatrix forms a prepreg. Combinations of both short and/or chopped fibersand continuous fibers may be utilized. The fibers may be sized orunsized.

Structural reinforcement fibers are typically at a concentration of 5 to35; preferably at least 20% by weight of the prepreg. For structuralapplications, it is preferred to use continuous fiber for example glassor carbon, especially at 30 to 70, more especially 50 to 70% by volume.

The structural reinforcement fiber can be organic, especially of stiffpolymers such as poly paraphenylene terephthalamide, or inorganic. Amonginorganic fibers glass fibers such as “E” or “S” can be used, oralumina, zirconia, silicon carbide, other compound ceramics or metals. Avery suitable reinforcing fiber is carbon, especially as graphite.Graphite fibers which have been found to be especially useful in theinvention are those supplied by Cytec Carbon Fibers under the tradedesignations T650-35, T650-42, T40/800 and T300; those supplied by Torayunder the trade designations T300, T700, T800-HB, and T1000; and thosesupplied by Hexcel under the trade designations AS4, AU4, AS7, IM 8 andIM 7, and HTA; and those supplied by Tobo-Tenax under the tradedesignations HTA, HTS and IMS fibers.

Organic or carbon fiber is preferably unsized or is sized with amaterial that is compatible with the curable composition according tothe invention, in the sense of being soluble in the liquid curablecomposition without adverse reaction or of bonding both to thestructural reinforcement fiber and to the thermoset/thermoplasticcurable composition. In particular, carbon or graphite fibers that areunsized or are sized with epoxy resin precursor are preferred. Inorganicfiber preferably is sized with a material that bonds both to thestructural reinforcement fiber and to the curable composition.

The curable resin matrix composition may be any suitable resin known foruse in the art that is preferably a liquid or a paste at ambienttemperatures and is preferably selected in conjunction with theselection of material for use as the resin-soluble thermoplastic polymerso as to completely dissolve the resin-soluble thermoplastic polymerwhen elevated to a temperature that is less than or equal to the curingtemperature for the resin, but below the melt temperature of theresin-soluble thermoplastic polymer. Thermosetting resins areparticularly preferred.

Preferably, the curable resin matrix composition is an epoxy resin andcan be selected from any known epoxy resin suitable to be infused into astructural reinforcement fabric for composite manufacture.

The curable resin matrix composition is preferably selected from thegroup consisting of an epoxy resin, an addition-polymerization resin,especially a bis-maleimide resin, a formaldehyde condensate resin,especially a formaldehyde-phenol resin, a cyanate resin, an isocyanateresin, a phenolic resin, polyester resins, vinylester resins andmixtures of two or more thereof, and is preferably an epoxy resinderived from the mono or poly-glycidyl derivative of one or more of thegroup of compounds consisting of aromatic diamines, aromatic monoprimaryamines, aminophenols, polyhydric phenols, polyhydric alcohols,polycarboxylic acids, cyanate ester resin, benzimidazole, polystyrylpyridine, polyimide or phenolic resin and the like, or mixtures thereof.Examples of addition-polymerization resin are acrylics, vinyls,bis-maleimides, and unsaturated polyesters. Examples of formaldehydecondensate resins are urea, melamine and phenols.

More preferably the curable resin matrix composition comprises at leastone epoxy, cyanate ester or phenolic resin precursor, which is liquid atambient temperature for example as disclosed in EP-A-0311349,EP-A-0365168, EP-A-91310167.1 or in PCT/GB95/01303. Preferably thethermoset is an epoxy or cyanate ester resin or a mixture thereof.

An epoxy resin may be selected from N,N,N′N′-tetraglycidyl diaminodiphenylmethane (e.g. “MY 9663”, “MY 720” or “MY 721” sold byCiba-Geigy) viscosity 10-20 Pa s at 50° C.; (MY 721 is a lower viscosityversion of MY 720 and is designed for higher use temperatures);N,N,N′,N-tetraglycidyl-bis(4-aminophenyl)-1,4-diiso-propylbenzene (e.g.Epon 1071 sold by Shell Chemical Co.) viscosity 18-22 Poise at 110° C.;N,N,N′,N′-tetraglycidyl-bis(4-amino-3,5-dimethylphenyl)-1-,4-diisopropylbenzene,(e.g. Epon 1072 sold by Shell Chemical Co.) viscosity 30-40 Poise at110° C.; triglycidyl ethers of p-aminophenol (e.g. “MY 0510” sold byCiba-Geigy), viscosity 0.55-0.85 Pa s at 25° C.; preferably of viscosity8-20 Pa at 25° C.; preferably this constitutes at least 25% of the epoxycomponents used; diglycidyl ethers of bisphenol A based materials suchas 2,2-bis(4,4′-dihydroxy phenyl) propane (e.g. “DE R 661” sold by Dow,or “Epikote 828” sold by Shell), and Novolak resins preferably ofviscosity 8-20 Pa s at 25.degree. C.; glycidyl ethers of phenol Novolakresins (e.g. “DEN 431” or “DEN 438” sold by Dow), varieties in the lowviscosity class of which are preferred in making compositions accordingto the invention; diglycidyl 1,2-phthalate, e.g. GLY CEL A-100;diglycidyl derivative of dihydroxy diphenyl methane (Bisphenol F) (e.g.“PY 306” sold by Ciba Geigy) which is in the low viscosity class. Otherepoxy resin precursors include cycloaliphatics such as3′,4′-epoxycyclohexyl-3,-4-epoxycyclohexane carboxylate (e.g. “CY 179”sold by Ciba Geigy) and those in the “Bakelite” range of Union CarbideCorporation.

A cyanate ester resin may be selected from one or more compounds of thegeneral formula NCOAr(YxArm)qOCN and oligomers and/or polycyanate estersand combinations thereof wherein Ar is a single or fused aromatic orsubstituted aromatics and combinations thereof and therebetween nucleuslinked in the ortho, meta and/or para position and x=0 up to 2 and m andq=0 to 5 independently. The Y is a linking unit selected from the groupconsisting of oxygen, carbonyl, sulphur, sulphur oxides, chemical bond,aromatic linked in ortho, meta and/or para positions and/or CR₂ whereinR₁ and R₂ are hydrogen, halogenated alkanes, such as the fluorinatedalkanes and/or substituted aromatics and/or hydrocarbon units whereinsaid hydrocarbon units are singularly or multiply linked and consist ofup to 20 carbon atoms for each R₁ and/or R₂ and P(R₃R₄R′₄R₅) wherein R₃is allyl, aryl alkoxy or hydroxy, R′₄ may be equal to R₄ and a singlylinked oxygen or chemical bond and R₅ is doubly linked oxygen orchemical bond or Si(R₃R₄R′₄R₆) wherein R₃ and R₄, R′₄ are defined as inP(R₃R₄R′₄R₅) above and R₅ is defined similar to R₃ above. Commerciallyavailable cyanate esters include cyanate esters of phenol/formaldehydederived Novolaks or dicyclopentadiene derivatives thereof, an example ofwhich is XU71787 sold by the Dow Chemical Company, and low viscositycyanate esters such as L10 (Lonza, Ciba-Geigy, Bisphenol derived).

A phenolic resin may be selected from any aldehyde condensate resinsderived from aldehydes such as methanal, ethanal, benzaldehyde orfurfuraldehyde and phenols such as phenol, cresols, dihydric phenols,chlorphenols and C₁-9 alkyl phenols, such as phenol, 3- and 4-cresol(1-methyl, 3- and 4-hydroxy benzene), catechol (2-hydroxy phenol),resorcinol (1,3-dihydroxy benzene) and quinol (1,4-dihydroxy benzene).Preferably phenolic resins comprise cresol and novolak phenols.

Suitable bismaleimide resins are heat-curable resins containing themaleimido group as the reactive functionality. The term bismaleimide asused herein includes mono-, bis-, tris-, tetrakis-, and higherfunctional maleimides and their mixtures as well, unless otherwisenoted. Bismaleimide resins with an average functionality of about twoare preferred. Bismaleimide resins as thus defined are prepared by thereaction of maleic anhydride or a substituted maleic anhydride such asmethylmaleic anhydride, with an aromatic or aliphatic di- or polyamine.Examples of the synthesis may be found, for example in U.S. Pat. Nos.3,018,290, 3,018,292, 3,627,780, 3,770,691 and 3,839,358. The closelyrelated nadicimide resins, prepared analogously from a di- or polyaminebut wherein the maleic anhydride is substituted by a Diels-Alderreaction product of maleic anhydride or a substituted maleic anhydridewith a diene such as cyclopentadiene, are also useful. As used hereinand in the claims, the term bismaleimide shall include the nadicimideresins.

Preferred di- or polyamine precursors include aliphatic and aromaticdiamines. The aliphatic diamines may be straight chain, branched, orcyclic, and may contain heteroatoms. Many examples of such aliphaticdiamines may be found in the above cited references. Especiallypreferred aliphatic diamines are hexanediamine, octanediamine,decanediamine, dodecanediamine, and trimethylhexanediamine.

The aromatic diamines may be mononuclear or polynuclear, and may containfused ring systems as well. Preferred aromatic diamines are thephenylenediamines; the toluenediamines; the various methylenedianilines,particularly 4,4′-methylenedianiline; the naphthalenediamines; thevarious amino-terminated polyarylene oligomers corresponding to oranalogues to the formula H₂N—Ar[X—Ar]_(n)NH₂, wherein each Ar mayindividually be a mono- or polynuclear arylene radical, each X mayindividually be —O—, —S—, —CO₂, —SO₂—, —O—CO—, C₁-C₁₀ lower alkyl,C₁-C₁0 halogenated alkyl, C₂-C₁₀ lower alkyleneoxy, aryleneoxy,polyoxyalkylene or polyoxyarylene, and wherein n is an integer of fromabout 1 to 10; and primary aminoalkyl terminated di- and polysiloxanes.

Particularly useful are bismaleimide “eutectic” resin mixturescontaining several bismaleimides. Such mixtures generally have meltingpoints which are considerably lower than the individual bismaleimides.Examples of such mixtures may be found in U.S. Pat. Nos. 4,413,107 and4,377,657. Several such eutectic mixtures are commercially available.

Preferably the resin-soluble polymer element and dissolving matrix areselected as a “solution pair” providing not only dissolution at desiredtime and temperature, but also good matrix injection, dispersion,morphology such as phase separation and traceless dispersion if desired,and the like. Suitable solution pairs include a low viscosity curableresin matrix composition for good injection and rapid dissolution, andcompatibility with the resin-soluble polyer element. Alternatively oradditionally less compatible resins may be used if it is desired tointroduce phase separation for enhanced mechanical properties.Combinations of different viscosity resins may be used each contributingseveral of the above properties where these are not provided by a singleresin.

A curing agent may also be introduced in combination with the presentinvention such as any known thermoset curing agent, for example epoxycuring agents, as disclosed in EP-A-0 311 349, EP-A 91310167.1, EP-A-0365 168 or in PCT/GB95/01303, which are incorporated herein byreference, such as an amino compound having a molecular weight up to 500per amino group, for example an aromatic amine or a guanidinederivative. Particular examples are 3,3′- and4-,4′-diaminodiphenylsulphone, (available as “DDS” from commercialsources),methylenedianiline,bis(4-amino-3,5-dimethylphenyl)-1,4diisopropylbenzene(available as EPON 1062 from Shell Chemical Co);bis(4-aminophenyl)-1,4-diisopropylbenzene (available as EPON 1061 fromShell Chemical Co); 4-chlorophenyl-N,N-dimethyl-urea, eg Monuron;3,4-dichlorophenyl-N,N-dimethyl-urea, eg Diuron and dicyanodiamide(available as “Amicure CG 1200 from Pacific Anchor Chemical). Otherstandard epoxy curing agents such as aliphatic diamines, amides,carboxylic acid anhydrides, carboxylic acids and phenols can be used ifdesired. If a novolak phenolic resin is used as the main thermosetcomponent a formaldehyde generator such as hexamethylenetetraamine (HMT)is typically used as a curing agent.

A catalyst may be introduced into a preferred embodiment of the presentinvention. In this case, the curing catalyst employed preferablycomprises a Lewis acid having amine functionality, instead of inaddition to conventional catalysts.

Preferably the catalyst is of the formula:LX_(n)R

-   -   where LX_(n) is a Lewis acid and R is an amine. Preferably L is        selected from Groups IIb, IIIb, VIII of the Periodic Table of        the Elements and X is a halogen.

Preferred catalysts include BF₃, BCl₃,AlF₃, FeF₃, ZnF₂ as Lewis acidcomponent and primary or secondary aliphatic or aromatic amine such asmonoethyl amine (mea), dimethylamine (dma), benzylamine (bea) orpiperidine.

A composite may be manufactured using the present invention, preferablythrough resin infusion obtained by a method known in the art wherein asupport structure comprising structural reinforcement fibers (dry) andthe at least one resin-soluble thermoplastic veil element is placed intoa bag, mold or tool to provide a preform and a curable resin matrixcomposition is injected/infused directly into the combined structuralreinforcement fibers and veil.

The preform is preferably formed, injected/infused and cured byprocessing techniques such as Resin Transfer Molding (RTM), Liquid ResinInfusion (LRI), Resin Infusion Flexible Tooling (RIFT), Vacuum AssistedResin Transfer Molding (VARTM), Resin Film Infusion (RFI) and the likeas hereinbefore referred.

Suitably the process includes a preliminary stage of infusion of curableresin matrix composition at reduced pressure, followed by a degassingstage drawing off air for reducing voidage. Traditionally the degassingis carried out under elevated pressure.

The resin-soluble thermoplastic veil element remains in solid formduring the initial stages of resin infusion and degassing, at ambient orreduced pressure, whereafter the polymer fibers dissolve and dispersewithout trace allowing the fluid phase components to compact, withoutexternal applied pressure, prior to onset of gelling coalescence andcuring. If external pressure is applied, the performance is simplyenhanced, however it is a particular advantage that this configurationallows for curing large panels without the need for an autoclave or thelike.

Suitably resin-soluble thermoplastic fibers are present in an amount of2 to 50 wt %, preferably 2 to 40 wt %, more preferably 4 to 16 wt % inthis embodiment.

Additionally the configuration has advantages during resin infusion,whereby channels remain open and clear to assist in rapid and uniformresin infusion throughout the panel.

Suitably, the process of the invention comprises subjecting to elevatedtemperature in the range up to 300° C. for example 60° C. to 200° C.,more preferably 75° C. to 150° C. for a period of up to 45 minutes,preferably 0.5 to 35 minutes to effect phase transition. Temperatures inthe range 100° C.-150° C. are particularly suitable for phase transitionof readily soluble polymer elements for example of low MW, present inreadily soluble concentration in an effective curable component solvent,and in the range 150° C. to 300° C. for less readily soluble polymerelements. Suitable elevated temperature is selected in a desired rangeto effect phase transition in a desired time, for example a givenflexible polymer element may be subjected to elevated temperature in therange 135° to 170° C. for 2-10 minutes, 125 to 135° C. for 5-30 minutesor 105° to 125° C. for 10-40 minutes.

Phase transition may be at ambient or elevated pressure corresponding tothe desired injection, degassing and curing conditions.

The process includes subjecting to further elevated temperature afterphase transition to cause onset of gelling or curing. Gelling may be attemperatures in the range corresponding to pre-cure in known manner.Gelling is preferably followed by further elevated temperature cure, orthe gelled composition may be cooled for later curing, for example ifgel or cure is in an autoclave, or mold, the composition may be removedfrom the autoclave or mold and cure continued at ambient pressure inregular oven.

Gelling or curing is suitably carried out by known means at elevatedtemperature and pressure for a suitable period, including temperatureramping and hold as desired. A suitable gelling or cure cyclecorresponds to that for a conventional composition comprising the samecomponent types and amounts and reference is made to the description andexample illustrating calculation of amount of flexible polymer elementpresent in the composition.

Preferably cure is at temperature in the range 150° C. to 400° C. for1-4 hours, for example. Additionally the process may include post curingat suitable conditions to enhance properties such as Tg and the like.

Gelling or curing may be with use of catalysts as hereinbefore defined,whereby temperature increase causes activation, and cooling belowactivation temperature halts curing.

The process may be monitored in real time but preferably a suitablereaction time and temperature is predetermined for a given composition,for example by preparing samples and analyzing solution and dispersionafter completion of gelling or cure, for example by use of Ramanspectroscopy or the like.

The invention is now illustrated in non-limiting manner with referenceto the following Examples.

EXAMPLES Example 1 Veil Manufacturing

54 tex fibers made from a PES polymer are chopped to an average fiberlength of 8 mm using a rotary blade chopper. The chopped fibers are thendispersed by hand on a release film and pressed between a press hotplatens at 180° C. for 1 hour with a 15 bar piston pressure. The platensare then cooled down to 60° C. and the resulting product is extracted.The product has a dimension of 15×15″ and an AW of 60 gsm. The veil isstable and handleable for processing purposes during manufacture.

Example 2 Veil Dissolution

A sample of veil made in the previous example (AW=60 gsm) weighing 10grams is combined with 30 g of Cycom™ 977 20 resin by dipping the veilinto the resin. The veil/resin combination is then placed in an oven at130° C. for 1 hour. The resulting product is then observed under amicroscope. The microscopic observation shows a homogeneous solutionwithout visible traces of undissolved fibers or high concentration ofthermoplastic.

Example 3 Composite Manufacturing

Layers of a HTA-6K-370 GSM 5HS carbon fabric were interleafed withnon-woven fabrics to manufacture RTM panels with a Vf=55% and a targetthermoplastic content of 25% wt in the resin (54 gsm in the fabric).FIG. 4 illustrates this resin infusion process while FIG. 5 illustratesthe veil and reinforcement fiber orientation.

The results show that the insertion of the non-woven veil providescompression after impact (CAI) testing numbers equivalent to traditionaltoughened prepreg systems using the non-woven resin-soluble veil. Thekey mechanical properties were tested and the results are reported inTable 1 below:

TABLE 1

Example 4 Stabilization of a Preform

A resin-soluble thermoplastic PES polymer veil was adhered to a carbonreinforcement fabric by contacting the PES polymer veil to the carbonreinforcement fiber to form a stabilized preform for resin infusion.

A layer of PES polymer veil and a layer of carbon reinforcement fabricwere fed between pressure rollers and a heat plate. FIG. 6 illustratesthis process for stabilization of the preform using the thermoplasticpolymer veil of the present invention. The line speed of the process wasabout 1.7 ft/min, the heat rollers were at a temperature, T₁, of about163° C.; and the chill roller was at a temperature, T₂, of about 21° C.All of the rollers had an applied pressure, P₁, P₂, and P₃,respectively, of 100 psi.

This process provided in-plane stabilization of the preform carbonfiber/thermoplastic polymer veil and allowed for subsequent heatbonding, stabilization, and preforming of dry fabric stacks prior tocomposite manufacturing using resin infusion techniques.

What is claimed is:
 1. A thermoplastic toughening element for resininfusion manufacturing of composite consisting of a layer of a pluralityof resin-soluble thermoplastic fibers randomly oriented and adhered toone another into a veil consisting of the resin soluble thermoplasticfibers with an areal weight of about 5 to about 80 gsm wherein thetoughening element is interposed between and in direct contact withadjacent plies consisting of a structural reinforcement fiber whereinthe resin-soluble thermoplastic fibers are in a solid phase and undergoat least partial phase transition to a fluid phase on contact with acurable resin matrix in which the thermoplastic fibers are soluble at afirst temperature for substantial dissolution, which is less than asecond temperature for substantial onset of curing of the curable resinmatrix, and wherein the concentration of toughening element is highestin the resin matrix between the adjacent plies and decreases inconcentration as the matrix approaches the plies.
 2. The tougheningelement of claim 1 wherein the fibers comprise a first resin-solublethermoplastic material and a second resin-soluble thermoplasticmaterial.
 3. The toughening element of claim 2 wherein the firstresin-soluble thermoplastic material and the second resin-solublethermoplastic material are the same.
 4. The toughening element of claim2 wherein the first material and the second material are different. 5.The toughening element of claim 1 wherein the resin-solublethermoplastic is selected from the group consisting of cellulosederivatives, polyester, polyamide, polyimide, polycarbonate,polyurethane, poly(methyl methacrylate), polystyrene, polyaromatics;polyesteramide, polyamideimide, polyetherimide, polyaramide,polyarylate, polyacrylate, poly(ester) carbonate, poly(methylmethacrylate/butyl acrylate), polysulphone, polyethersulphone,polyetherethersulphone polyethersulphone-etherketone, and copolymers andcombinations thereof.
 6. The toughening element of claim 1 wherein thefibers solubilize in a curable resin matrix at a first temperature lessthan a second cure temperature of the curable resin matrix.
 7. Thetoughening element of claim 6 wherein the solubilized fibers chain linkwith the curable resin matrix at the second cure temperature of thecurable resin matrix.
 8. The toughening element of claim 1 wherein thefibers carry modifiers.
 9. A toughening element for a curablecomposition consisting of a veil consisting of resin solublethermoplastic fibers in a solid phase interposed between and in directcontact with layers consisting of reinforcement fibers wherein thetoughening element undergoes at least partial phase transition to afluid phase on contact with a curable composition in which it issoluble, at a first temperature for substantial dissolution, which isless than a second temperature for substantial onset of gelling and/orcuring of the curable composition and which first temperature is lessthan a third temperature for substantial melting of the resin-solublethermoplastic veil, and wherein the concentration of toughening elementis highest in the resin matrix between the adjacent plies and decreasesin concentration as the curable composition approached the layers ofreinforcement fibers.
 10. The toughening element of claim 9 wherein thelayers of reinforcement fibers form a preform and wherein theresin-soluble thermoplastic veil stabilizes the preform.
 11. A preformfor composite manufacturing comprising: a structural componentconsisting of a plurality of structural reinforcement fiber plies; atoughening element consisting of a plurality of resin-solublethermoplastic fibers randomly oriented and adhered to one another into aveil interposed between and in direct contact with adjacent plies ofstructural reinforcement fibers; wherein the toughening element is inthe form of a non-woven veil with an areal weight of about 5 to about 80gsm in contact with the adjacent plies of structural reinforcement fiberin the structural component, wherein the toughening element is in asolid phase and substantially remains in a soluble phase when contactedwith a curable resin matrix at a first temperature for infusion, whichis less than a second temperature for substantial dissolution of thetoughening element, and wherein the concentration of toughening elementis highest in the resin matrix between the adjacent plies and decreasesin concentration as the matrix approaches the plies.
 12. The preform forcomposite manufacturing of claim 11 wherein the structural component isprovided in the form of a plurality of adjacent reinforcement fiberlayers and wherein the toughening element is provided in the form of aplurality of veils interposed between pairs of adjacent reinforcementfiber layers.
 13. The preform for composite manufacturing of claim 11adapted for resin infusion.
 14. The preform for composite manufacturingof claim 11 wherein the preform is stabilized.
 15. The preform forcomposite manufacturing of claim 11 wherein the toughening element is ina solid phase and substantially remains in a soluble phase whencontacted with a curable resin matrix at a first temperature forinfusion, which is less than a second temperature for substantialdissolution of the toughening element.
 16. The preform for compositemanufacturing of claim 11 wherein the toughening element substantiallyreacts with the curable resin matrix at the second temperature.
 17. Thetoughening element of claim 1 wherein the adjacent plies of a structuralreinforcement fiber are in the form of a woven or fabric structure. 18.The toughening element of claim 1 wherein the adjacent plies of astructural reinforcement fiber are in the form of unwoven orunidirectional fibers.